Modern pyro-metallurgical furnaces and especially blast furnaces must enclose heat so intense that the refractory crucible must be aggressively liquid-cooled. Very high temperatures are needed to reduce and melt iron ore and produce a high-carbon cast iron (pig iron).
Refractory brick linings wear very rapidly when they run too hot. So stave coolers are employed to keep the refractory brick lining temperatures under control, and wear to a minimum.
Vertical shaft blast furnaces vary in height from 24-33 meters and have hearth diameters of about 8.5 meters and are widely used in iron making. A typical blast furnace's volume runs more than 1400 m3. Blast furnaces have charging arrangements at the top and a means of running off the pig iron and slag at the bottom. Hot air is blown in through tuyeres near the bottom of the furnace, which burns coke to produce carbon dioxide (CO2). This then further reduces Fe2O3 to produce Fe+CO2.
Conventional stave cooler and cooling panel designs have typically installed their refractory brick into grooves on the hot faces before installing the panels themselves inside the furnace shell. Many brick designs are intended to be installed on flat/parallel panels. One brick design was able to be installed in flat/parallel and curved coolers after the cooler was in place. When a furnace needs rework, such bricks that can be replaced or re-installed without removing the cooler offer a distinct advantage.
Stave coolers with pre-installed bricks are installed in the furnace with a gap in between them to allow for construction variances and thermal expansion allowance. Such gaps must be filled with mortar, castable, or rammed refractory to close the openings. Unfortunately, this fill material can be lost if the staves are not designed and installed properly.
The ram gaps erode during operation and furnace gases leak through between the staves. Prior attempts have been made to brick continuously around the furnace's circumference to eliminate the filled gaps. Hopefully increasing the integrity and life of the furnace. Any edges left protruding into the furnace are exposed to catching churning matter in the process.
Any exposed edges tend to wear rapidly and can cause the bricks to crack and break off. Missing brick or fill refractory exposes the stave to more serious damage.
Any good brick must be fully installable in tilted or angled walls. Conventional stave and cooling panel bricks were typically installed in straight grooves to keep the bricks in the coolers. Tapered bricks which were not locked into the grooves instead pushed against the cooler. Stave grooves can be used to lock-in the brick, and are tapered from back to front to key the brick in place. When the bricks inflate under heat, the tapered shapes help them push out against the cooler and reduce their thermal resistance.
Some staves have been installed without refractory brick in front of them. There may have been an initial coating of castable refractory for startup. These tried to freeze a skull layer for protection and insulation when operations began a blast furnace. Such skull is generated and spalled repeatedly in service. Each cycle of loss and regeneration leads to increased temperatures and stresses in the stave, eventually leading to cracking and the possibility of losing cooling fluid into the furnace. (Which can result in a powerful and very destructive steam explosion.)
Bricks that stay in place keep stave surface temperatures stable and more uniform. This allows for more consistent furnace operation and less heat loss and longer service life for the stave.
Skulls will only form in the cohesive zones of a furnace. So, a skull approach is not effective if the cohesive zone is not correctly determined. Unfortunately, the cohesive zones of furnaces can change with the charge material. Here too, brick refractory linings better protect the staves regardless of adhesion.
It is nevertheless appropriate to form a skull to protect the refractory similar to as in a basic oxygen furnace. Skull adhesion is lost in various sections in the furnace at different times. This results in non-uniform temperatures throughout the staves and furnace. A continuous circumferential brick pattern built around inside a furnace, and locked into the staves, can use thermal expansion to increase contact, and thereby maintain a uniform stave temperature. Uniform stave temperatures are good because they reduce stresses on both the furnace and staves, and both enjoy longer lives.
Some early locked-in brick designs were relatively thin, and so these bricks would crack easily and fall through into the furnace. Better bricks must increase thickness for better strength, and make them less susceptible to cracking.
In a so-called double-lock system, a keystone type taper to the sides was expected to hold broken bricks in place. Increased thickness was also predicted to allow the bricks to be installed faster. Any additional weight to the brick tended to keep them in place better and less susceptible to failure. Many older stave designs which put bricks in walls in front of the staves needed many bricks. The joints between them got in the way of effective cooling, especially cooling of those bricks furthest from the stave cooler. Artisans later found it better to incorporate only one brick in tight contact with the stave coolers, e.g., to eliminate any thermal barriers that multiple mortar joints would introduce.